THE NERVOUS SYSTEM OF VERMES. 



as to form three or four nerve-trunks on each side, which 

 enter a comparatively large compound ganglionic mass 

 (a, a) lying on the lateral aspects of the sheath of the 

 proboscis. Each of these masses is pyriform in shape, and 



composed of a sensory and 

 a motor ganglion fused into 

 one. It is connected with 

 its fellow by means of two 

 commissures, one of which 

 passes over, and the other 

 beneath, the proboscis. 



It is difficult to trace the 

 ultimate distribution of the 

 nerve-fibres in these crea- 

 tures ; so that, although 

 fibres can be followed nearly 

 up to the pigment-spots, 

 none have been detected in 

 immediate continuity there- 

 with. The inferior com- 

 missure (c) between the two 

 ganglionic masses is shorter 

 and much thicker than the 

 upper. The two great late- 

 ral nerve-trunks (d, d) start 

 from the ganglia, and, pro- 

 ceeding along the sides of the body, give off numerous 

 branches to the longitudinal and circular muscles between 

 which they are situated. 



Tactile and possibly gustatory impressions, together 

 with impressions produced by light or darkness, doubtless 

 come from the anterior extremity of the organism to the 

 anterior part of the pyriform ganglia on either side ; and 

 are thence reflected from the posterior parts of these bodies 



FIG. 30. Head and Brain of a Nemertean. 

 (Tetrastemma melanocephala.) a, a, Com- 

 pound lateral ganglia ; b, narrow upper 

 commissure between which and the much 

 thicker inferior commissure, c, the cesopha- 

 gus passes ; d, d, the great lateral nerve 

 cords ; e, e, pigment spots, or rudimentary 

 ocelli. (After Mclntosh.) 



